Conventional linearly polarizing elements, such as linearly polarizing lenses for sunglasses and linearly polarizing filters, are typically formed from stretched polymer sheets containing a dichroic material, such as a dichroic dye. Consequently, conventional linearly polarizing elements are static elements having a single, linearly polarizing state. Accordingly, when a conventional linearly polarizing element is exposed to either randomly polarized radiation or reflected radiation of the appropriate wavelength, some percentage of the radiation transmitted through the element will be linearly polarized.
In addition, conventional linearly polarizing elements are typically tinted. Typically, conventional linearly polarizing elements contain a coloring agent and have an absorption spectrum that does not vary in response to actinic radiation. The color of the conventional linearly polarizing element will depend upon the coloring agent used to form the element, and most commonly, is a neutral color (for example, brown or gray). Thus, while conventional linearly polarizing elements are useful in reducing reflected light glare, because of their tint, they are typically not well suited for use under low-light conditions.
Conventional linearly polarizing elements are typically formed using sheets of stretched polymer films containing a dichroic material. Correspondingly, while dichroic materials are capable of preferentially absorbing one of two orthogonal plane polarized components of transmitted radiation, if the molecules of the dichroic material are not suitably positioned or arranged, no net linear polarization of transmitted radiation will be achieved. Without intending to be bound by any theory it is believed that due to the random positioning of the molecules of the dichroic material, selective absorption by the individual molecules will cancel each other such that no net or overall linear polarizing effect is achieved. As such, it is typically necessary to position or arrange the molecules of the dichroic material by alignment with another material so as to achieve a net linear polarization.
A common method of aligning the molecules of a dichroic dye involves heating a sheet or layer of polyvinyl alcohol (“PVA”) to soften the PVA and then stretching the sheet to orient the PVA polymer chains. Thereafter, the dichroic dye is impregnated into the stretched sheet, and the impregnated dye molecules adopt the orientation of the polymer chains. Resultantly, at least some of the dye molecules become aligned, such that the long axis of each aligned dye molecule is generally parallel to the oriented polymer chains. Alternatively, the dichroic dye can be first impregnated into the PVA sheet, and thereafter the sheet can be heated and stretched as described above to orient the PVA polymer chains and associated dye. In this manner, the molecules of the dichroic dye can be suitably positioned or arranged amongst the oriented polymer chains of the PVA sheet, and a net linear polarization can be correspondingly achieved. As a result, the PVA sheet can be made to linearly polarize transmitted radiation, and correspondingly a linearly polarizing filter can thus be formed.
In contrast to the dichroic elements discussed above, conventional photochromic elements, such as photochromic lenses that are formed using conventional thermally reversible photochromic materials are generally capable of converting from a first state, for example a “clear state,” to a second state, for example a “colored state,” in response to actinic radiation, and reverting back to the first state in response to thermal energy. Thus, conventional photochromic elements are generally well suited for use in both low-light and elevated-light (or bright) conditions. Conventional photochromic elements, however, that do not include linearly polarizing filters are generally not capable of linearly polarizing incident radiation, such as direct or reflected light. The dichroic ratio of conventional photochromic elements, in either state, is generally less than two. Therefore, conventional photochromic elements are not capable of reducing reflected light glare to the same extent as conventional linearly polarizing elements. In addition, conventional photochromic elements have a limited ability to store or display information.
Photochromic-dichroic compounds and materials have been developed that provide both photochromic properties and dichroic properties, if properly and at least sufficiently aligned. Photochromic-dichroic compounds can be aligned in accordance with the stretching methods described above with regard to dichroic dyes, and/or other methods, such as electromagnetic alignment methods. Some current photochromic-dichroic compounds, however, can be subject to reduced stability. For example, some current photochromic-dichroic compounds can have poor fatigue resistance. Photochromic-dichroic compounds and materials having reduced fatigue resistance typically exhibit a reduced or degraded photochromic response after extended exposure to actinic radiation. A reduced or degraded photochromic response can involve an undesirable increase in the amount of time required for the photochromic-dichroic compound or material to transition between a first state and a second state, such as from a colored state to a non-colored state, and visa versa.
It would be desirable to develop new photochromic-dichroic compounds that provide a desirable combination of photochromic properties and dichroic properties. In would be further desirable that such newly developed photochromic-dichroic compounds also posses improved stability, such as improved fatigue resistance.